MOSCOW — Russian military officials declared partial victory Saturday in their four-day-old bombing campaign in Syria and said they would intensify airstrikes against rebel groups despite U.S. criticism.

Officials in Washington said Russia was stepping up its military support to the government of Syrian President Bashar al-Assad, sending at least two multiple-rocket launcher systems through the Russian naval base at Tartus on Syria’s coast. Such systems can blanket a large area with munitions.

Russian aircraft based in Syria had carried out 20 sorties in 24 hours and destroyed nine targets, the Russian Defense Ministry said in a statement Saturday morning. They included strikes near the city of Raqqa, which is controlled by the Islamic State, and in the provinces of Idlib and Hama, which are not.

Activists and human rights groups said the strikes continued for a fourth consecutive day, with the attacks extending into northern Latakia province, near the expanded air base from which the Russian air force is operating.

One village, Ihsem, in the Jabal al-Zawiya mountain area of Idlib, was hit by five successive raids, and a rescue worker belonging to the Western-funded White Helmets rescue service was killed. The village was seized from moderate rebels by al-Qaeda affiliate Jabhat al-Nusra last year and is now under the control of the Jaish al-Fatah, or Army of Conquest, a coalition of Islamist and rebel groups.

The Syrian Observatory for Human Rights, an activist and monitoring group based in Britain, said Russian airstrikes had killed 39 civilians in the previous four days. Russia has insisted that all its strikes have targeted the Islamic State, but most of the raids have taken place in areas from which the Islamic State was ejected more than a year and a half ago, and that are now controlled by a mixture of Western-backed moderate rebels, Islamists and al-Qaeda-linked fighters opposed to the Assad regime.

According to Western officials and footage of Russian airstrikes, some Russian aircraft are using unguided munitions, so-called “dumb bombs,” which are more likely to miss their targets and cause collateral damage.

Gen. Andrei Kartapolov on Saturday declared that four days of Russian airstrikes had “significantly decreased the fighting potential” of the Islamic State. “There is panic and desertion in their ranks,” Kartapolov told journalists. “More than 600 mercenaries have left their positions and are trying to get to Europe.”

The statement could not be confirmed, but the strikes appeared to have hardened resistance among government opponents to a U.N. initiative aimed at exploring potential solutions to the war.

In a rare display of unity, the Syrian Opposition Coalition and dozens of armed groups spanning a spectrum of views from radical Islamist to moderate issued a statement saying they would not attend a series of “working groups” the United Nations hopes to convene later in the fall to discuss the possibility of negotiations.

Here's what you need to know about Russia's airstrikes in Syria. The Russian military claims the strikes target the Islamic State, but U.S. officials say it's not helping. (Jason Aldag/The Washington Post)

Citing “the current popular outrage” against the lack of world response to Russian as well as Iranian intervention in Syria, the statement said any political process should explicitly declare that there is no future role for Assad.

The statement was signed by the Salafist-jihadist Ahrar al-Sham group as well as some of the rebel units that have received training and weapons from the United States and its allies.

Syria’s government has said it will attend the talks, but Syria’s foreign minister, Walid al-Moualem, also said Friday that Damascus will not countenance any reforms to the government or the constitution until after the war against “terrorism” is won.

Speaking Friday at the United Nations in New York, he warned Assad’s opponents not to expect to . “achieve at the negotiating table more than in the battlefield,” according to comments quoted by the Russian news agency Tass.

Sly reported from Beirut. Thomas Gibbons-Neff in Washington contributed to this report.